Method for dynamically controlling decarburization of steel

ABSTRACT

DESCRIBED HEREIN IN A METHOD OF DYNAMICALLY CONTROLLING DECARBURIZATION OF STEEL BY MEASURING THE RATE OF CARBON REMOVAL FROM THE STEEL, MEASURING THE RATE OF OXIDIZER INPUT, CONTINUOUSLY MAINTAINING A BALANCE BETWEEN THE AFOREMENTIONED RATES BY ADJUSTING THE CARBON-OXYGEN REATION RATE AND/OR ADJUSTING THE INPUT OF OXIDIZING MATERIAL TO THE STEEL.

July 20. 1971 s. RAMACHANDRAN 3,594,155

METHOD FOR DYNAMIC/ALLY CONTROLLING DECARBURI ZATION OF STEEL 2Sheets-Sheet 1 Filed Oct. 50, 1968 Qmu /NVE/V TOR SUNDAHESAN RAMACHA/VDRA/V @y /l/m uw@ lg A flor/my July 20. 1971 s. RAMACHANDRAN3,594,155

f METHOD FOR DYNAMICALLY CONTROLLING DECARBURIZATION OF STEEL Filed oct.3o, 196s DYNAM/CLLY CONTRO/.LED DECARBUR/ZAT/ON THROUGH TEMPERATUREVAR/A-T/ON Bar/1 Temperatura Carbon Conte/1l PROCESS TIME minules //vVEN Tof? SU/VDA RESA /v RA MA CHA/vom /v @y W/M Mw A flor/1 e y UnitedStates Patent O METHOD FOR DYNMCALLY CoNTRoLLrNG DECARBURIZATION F STEELSundaresan Ramachandran, Natrona Heights, Pa., assignor to AlleghenyLudlum Steel Corporation, Brackenridge, Pa.

Filed Oct. 30, 1968, Ser. N0. 771,752

Int. Cl. C21c 5/32 U.S. Cl. 75--60 6 Claims ABSTRACT OF THE DISCLOSUREDescribed herein is a method of dynamically controlling decarburizationof steel by measuring the rate of carbon removal from the steel,measuring the rate of oxidizer input, continuously maintaining a balancebetween the aforementioned rates by adjusting the carbon-oxygen reationrate and/ or adjusting the input of oxidizing material to the steel.

This invention relates to decarburizing steel. More particularly, theinvention relates to an improvement in the method of decarburizingmolten steel wherein oxidizing material is introduced to the steel toreact with carbon contained therein. Still more particularly theimprovement in accordance with the invention comprises a dynamic methodof controlling decarburization of molten steel by measuring the rates ofcarbon removal from the molten steel, oxidizer input rate, continuouslymaintaining a balance between the two rates by adjusting thecarbonoxygen reaction rate and/ or oxidizer input rate in response tothe measured rates. The term balance as used herein means adjusting therelationship between the rates of carbon removal from the steel andoxidizer input to achieve any desired result. For example, avoidance ofoxidation of expensive alloying elements is achieved by maintaining abalance such that the rate of oxidizer input is never greater than therate of carbon removal.

Decarburizing is an essential part of present day steel making practicesand more and more commonly is performed by blowing pure oxygen into themelt contained in a vessel or furnace, e.g. electric furnace, openhearth, basic oxygen furnace (BOF), etc. This so called oxygen steelmaking now is practiced in both the manufacture of plain carbon steeland alloy steel. The oxygen efficiency for decarburization processes ofthe type described can be defined as:

Percent oxygen efliciency: (oxygen to the decarburization reaction+netoxygen to the system) 100 This efficiency figure can be used to checkhow effectively the oxygen is used to remove carbon. Although the primepurpose of oxygen is the removal of carbon, it will also oxidizesilicon, phosphorus and, if not properly controlled, other metallicvalues. It is apparent that to make best use of oxygen, control of thefactors affecting oxygen eiiciency is necessary.

The present invention, which is useful in both vacuum and atmosphericpressure decarburization and for alloy steel as well as plain carbonsteel, involves dynamically balancing the rate of oxygen input and therate of carbon removal and adjustment of the carbon removal rate and/ oroxygen input rate to achieve any particular maximum oxygen eliciency orany other desired result. Thus, for example, by practicing the inventionit is possible to adjust the oxygen input so that the normalpreferential oxidation of carbon can be accomplished without loss ofmore valuable metal values such as chromium and other alloying elements.This is achieved by assuring that the total oxygen available duringdecarburization does not exceed that needed for carbon removal.

The carbon removal rate can be determined by any of several ways. Thusfor example, the composition of the bath can be continuously sampled andanalyzed for carbon to determine the quantity of carbon removed per unitof time. Another, and presently preferred method, is to monitor theexhaust gases from the reaction vessel and measure the total ow and theamounts of carbon monoxide and carbon dioxide in the off-gas stream suchas by the techniques described below. Analyses of off-gas compositionand measurement of flow can be used to determine the rate of carbonremoval almost instantaneously. This rate of carbon removal, which canbe expressed conveniently as pounds per minute, is equal to the volumeof carbon monoxide and carbon dioxide leaving the furnace at any givenmoment multiplied by a conversion factor. Set forth in an equation, thisrelationship is expressed as follows:

Lbs. of carbon removed/min.=(volurne percent CO-I- volume percent CO2 inthe offgas)-X flow rate of od-gas (standard cubic ft./min.) X (l2=atomicweight of carbon, 859=conversion factor based on fact 359 cubic feet ofa gas contains a mole of gas) Similarly, the volume of equivalent oxygenthat has reacted with this carbon at any time during the process may beobtained by the following equation which gives the rate at which oxygenis consumed by the carbon. For practical purposes this consumption rateis expressed as cubic feet per minute to correspond to the ilow rate ofthe off-gases.

Rate at which oxygen is consumed by the carbon (standard cubicfeet/min.)=1/2 [(vol. percent CO in off-gas) -l-vol. percent CO2 inoff-gas] flow rate of off-gas standard cubic feet/min.)

Whether the oxygen supplied to the vessel for decarburization is beingconsumed by the carbon in the melt or whether metallic elements arebeing oxodized can be determined by noting whether the rate of oxygenconsumed by the carbon is greater, equal to or less than the rate ofoxygen input. Adjustment of the oxygen input can be made in response tothe measured rates of carbon removal and oxygen consumption. Bycontinuously adjusting the variables responsible for the carbon-oxygenreaction and/or the flow rate of oxygen into the reaction vessel thedecarburization reaction may be continuously, i.e. dynamically,balanced.

The oxygen input rate and/or the carbon removal rate can be varied inaccordance with the measured rates of carbon removal and oxygenconsumption by varying the oxygen etliciency which amounts to alteringthe nate of the carbon-oxygen reaction. This can be varied by severalmeans, such as:

(l) Changing the oxygen input rate, e.g., by including a diluent gas orincreasing the proportion of diluent gas mixed with oxygen introduced tothe reaction vessel;

(2) Changing the pressure in the vessel;

(3) Changing the temperature of reaction; and

(4) Changing the reaction surface area, i.e. agitating or increasing themixing of oxygen and steel.

Change in the oxygen input rate may be accomplished by simply reducingthe flow rate of oxygen when pure oxygen is used. In this way, themixing caused by the input of the gas is reduced and the oxygenconsumption rate is also reduced. However, if the total gas ow rate ismaintained and a nonreactive gas, e.g. a diluent, is substituted foroxygen, the rate of oxygen reaction with carbon is not lowered. Thus,the oxygen input rate can be varied by including a diluent gas withoxygen, but maintaining the same total gas flow rate, `without reducingthe rate of oxygen reaction with carbon. Diluent gases 3 which may beused other than inert gases are, for example, hydrogen and carbonmonoxide as well as steam or carbon dioxide. Carbon dioxide and carbonmonoxide do not displace the equilibrium of the carbon-oxygen reactionin any way.

The amount of oxidizing material required for carbon removal in an inputgas stream can be calculated using the gaseous composition, the massflow rate and the stoichiometry of the reactions within the steel melt.The reaction of these oxidizing gases with carbon in the melt can bewritten as:

It is evident from the above that the same volume of carbon dioxide andsteam can combine with only half as much carbon as pure oxygen. Thus thetotal oxidizing material, i.e. gaseous input, in terms of oxygenequivalent, can be calculated by the following equation:

Volumetric flow rate of oxidizer (expressed as volumes of O2 per unittime)=volumetric flow rate of oxygen in input gas-I-l/z (sum of thevolumetric flow rates of carbon dioxide and steam in input gas) The rateof carbon removal must be corrected for the carbon input to the system.This correction can be obtained by measuring the volumetric rate atwhich carbon is fed into the system as carbon monoxide or carbondioxide. The carbon input can be expressed in terms of oxygen equivalentas follows:

Carbon input (volumetric rate in equivalent oxygen units)=1/2 (sum ofvolumetric flow rate of carbon monoxide and carbon dioxide in input gas)To complete the dynamic balance, the products of reaction, specicallythe amount of oxidizing material combined with carbon in the melt, mustbe estimated. One technique of the many that may be used, is todetermine the composition of the exhaust gases and the exhaust gas owrate. It may be assumed that the exhaust gases contain all of the inertor diluent gases and the gaseous products of reaction with the melt. Inaddition, the exhaust gases will also include the unreacted portions ofthe input gas and other gases entering the system. The reaction productscan be viewed as:

(Note: Instead of iron, any other metallic element can combine withsteam or carbon dioxide to form H2 or CO.)

The amount of oxygen needed for decarburization can be calculated fromthe composition and oW rate of the exhaust gas. The flow rate from theexhaust gas can either be estimated by means of a calibrated orificeplate or can be calculated using a tracer gas technique. In the latter,a tracer gas at a known flow rate can be mixed completely with theexhaust gas and the flow rate of the exhaust gas can be calculated.Whenever an inert gas such as argon is used in gas mixtures with oxygen,the inert gas, e.g. argon, can be used as the tracer gas and thevolumetric ow rate obtained as follows:

Volumetric'ow rate of exhaust gas lOOXvolurnetric flow rate of inputargon Volume percent argon in output str earn The presence of air leakswill affect the determination of volume flow rate when argon is usedsince air contains on the order of 0.94% argon by volume. A correctioncan be made where argon concentrations added by the air is discounted.

(volume percent N2 in exhaust gas):|

Volumetric flow rate of exhaust gas Volurnetric flow rate of steamX Sumof percentages of hydrogen and steam in exhaust gases After determiningthe flow rate of the exhaust gases the carbon removal rate can bemeasured. This can be done on an oxygen equivalent basis by thefollowing equation: A

Carbon removal rate (in volumetric flow rate in equivalent -02units)=1/z volumetric flow rate of exhaust gases (sum of volume percent0f CO and CO2)-car bon input To 11p-date and correct the determinationof oxygen required for carbon removal, it may be necessary to considersmall amounts of unreacted oxygen or steam that can leave the systemalong with other products of reactions which have not gone tocompletion. To provide such correction, the following relationship maybe used:

Corrected rate of oxygen required for the carbon removalzcarbon removalrate-|-volumetric exhaust gas ow rate (1/2 volume percent H2O-l-1/zvolume per cent COz-i-volume percent O2 in exhaust gases) The dynamicbalance between the actual total oxygen input rate and the correctedrate of oxygen required for carbon removal can be performed Ibycomparing the input and output rates. The input rate can be determinedaccording to the following expression:

Total oxygen input rate (volumetric flow units) :volumetric ow ofoxidizer input -I-[lOw rate of exhaust gases X (20.95 volumetric percentN2 in exhaust gas):|

As can be seen, the oxygen input rate should account for both thedeliberate input oxygen as well as accidental and incidental sources ofoxygen such as air or water leaks. Only the two variables, the totaloxidizer input rate and the corrected rate of oxygen required for carbonremoval are determined. These values can be compared to determinewhether the desired balance is maintained.

yIn decarburizing stainless steel, elements such as silicon and aluminummust, if present, be oxidized before the carbon level can be reduced tolow values. In such cases, the rate at which silicon, aluminum, etc.,are being oxidized can be measured and included in determining theoxygen input so that sufficient oxygen is provided to accomplishdecarburization at the desired rate as well as oxidation of theelements, e.g. silicon, aluminum, also intended to lbe removed. In sucha process the silicon loss rate can be estimated by noting thedifference between the rate of input of the oxidizing material and thecorrected oxygen rate required Ifor carbon removal. To determine whethermetallic oxidation is occurring, the total input rate of oxidizingmaterial may be compared with the estimated oxygen required for carbonremoval. 1f the oxygen input rate is greater than that required for thecarbon removal, it can b e concluded that metallic oxidation isoccurring. To restore the dynamic balance and avoid metallic oxidation,one or a combination of the following practices may be used.

(1) Lower the content of oxidizing material in the input stream Whilemaintaining its overall llow by increasing the volume of the diluent(the diluent could be one or more of the inert gases such as argon,steam, carbon monoxide or carbon dioxide).

(2) Lower the gas ilow rate in systems in which the pressure need not bemaintained (this technique will not normally be effective if the systempressure is maintained since the carbon removal rate will also belowered. The rate of carbon removal could remain the same even withlowered ow rate of input gases since the system pressure could becomelower and result in an increase in the driving force for the carbonremoval reaction),

(3) Increase the carbon removal rate by increasing the systemtemperature.

Restoration of balance in decarburization of plain carbon steels canalso be accomplished as described above. The essential differencebetween decarburizing alloy steels and decarburizing low carbon steelsis that the iron oxide buildup in the slag in plain carbon steel isdesirable for phosphorus removal. The efliciency of carbon removal maylbe desirably low at the start and then improved as the carbon contentis lowered. It is only near the end of the process that elimination ofiron loss would be particularly desirable. The efliciency of carbonremoval can be controlled by varying lance height or by controlling therate of additions of lime, ore, etc. Near the end of decarburization theuse of carbon monoxide and oxygen or even carbon dioxide and oxygen maybe preferred.

Another technique for determining the occurence of metallic oxidation islby the ratio f inert gases to Carbon-containing gases in the exhauststream. For example, where mixtures of argon and oxygen are used fordecarburization, it may be assumed that all of the input oxygen willreact with the carbon and the expected ratio of argon to carbon-bearinggases will be as follows:

Expected Ar/(CO-l-CO2) ratio percent Ar in input gas 2 (10D-percent Arin input gas) If the expected ratio is greater than the actual ratio,metallic oxidation will -be occurring. Corrections for air leaks mayalso be made to the ratio and if carbon dioxide or carbon monoxide isused in the input stream, adjustments for these components may also beincluded in the ratio. A similar program can be established for mixturesof steam and oxygen or hydrogen and steam.

Several recent patents discuss the use of a mixture of inert gas andoxygen, especially with reference to stainless steel melting. Amongthese are U.S. Pat. 3,003,865 which describes the use of inert gas andoxygen for decarburizing stainless steel and Pat. Nos. 3,046,107 and3,252,770, which describe how argon or other inert gases can be used toexercise some `degree of control of the decarburizing process. Thesepatents describe certain theoretical relations, based on thermodynamicequilibria, which are applicable to controlled decarburization ofstainless and alloy steels using oxygen and inert gas mixtures. Thus inU.S. Pat. 3,046,107 the maximum oxygen content of a mixture that can beemployed for decarburization, with negligible chromium loss, is givenby:

lPercent oxygen antilog (1378,00-8.46)] 1 Percent Cr Percent C degreesKelvin. A refinement of the above relationship is given in U.S. Pat.3,252,770:

1/4 Percent carbon=|: (percent C03] P where Kt is the thermodynamicequilibrium constant derived from the activities of carbon and chromiumat the melt temperature, and P is the pressure surrounding the melt.

By applying the above relationships, a theoretical gas supply scheduleand decarburization scheme can be derived, involving stepwise, or wherepossible, continuous reduction of the proportion `of oxygen in theinjected gas stream as the carbon content of the bath is lowered. Anillustration of the application of this technique is the followingexample of a l5 ton heat processed using the aforementionedrelationships. The chemical composition of the melt beforedecarburization was as follows: C=0.95%; Mn=0.93%; S=0.0l2%; Si=0.82%;

Ni: 12.17%. An oxygen (68.1% by volume) and argon mixture was injectedvia subsurface means into the melt for forty (40) minutes, and at theend of this step, the carbon and chromium contents were 0.179% and17.32% respectively. Subsequently, a 38.5% oxygen-argon mixture wasinjected for an additional seventeen (17) minutes, at the end of whichperiod, the carbon and chromium levels had decreased to 0.045% and16.65% respectively. During the process, the melt temperature rose froman initial value of 2820 F. to 3140" F., and 1.77% chromium was lost byoxidation.

It should be observed that according to -the lprovisions of theequations presented previously, the carbon content in equilibrium withthe initial chromium in the bath (assuming an average melt temperatureof 3000" F.) when a I68.1% oxygen-argon mixture is employed is 0.259%.During decarburization from the initial level of 0.95% to thisequilibrium value, carbon Was preferentially oxidized. Bu-t since gasinjection with the 68.1% oxygen mixture proceeded beyond this point,.both carbon and chromium were oxidized simultaneously as soon aS carbondropped below 0.259%, and hence the observed chromium loss (to 17.32%).Since, according to the practice described in U.S. Pat. 3,046,107, for a38.5% oxygen-inert gas mixture, and for a 17.32% chromium content, theequilibrium carbon at 3000 F. in 0.105 preferential oxidation of carbonwas resumed during the second step. However, when decarburizationproceeded beyond 0.105% carbon, chromium oxidation again set in and bythe end of the process, this element had been reduced to 16.65%. Thus,practicing the embodiments of the previously cited patents does notguarantee negligible chromium loss. This is in part due to the inabilityto recognize the onset of chromium oxidation and to adjust the gas blowaccordingly.

The embodiments of the present invention, based on the maintenance of adynamic balance between the input oxygen and the off-gases from thedecarburization process, provide techniques for the accomplishment ofdecarburization without chromium loss. The application of the inventionrequires close regulation of, among others, the following parameters:

(a) Input and entrained gas composition (b) Ambient pressure around themelt (c) Melt temperature (d) Gas-metal Contact area In addition, meansshould be available for measurement and alteration of each of the abovequantities. Several means for achieving these objectives are discussedhereinafter with accompanying examples. In all cases, means forinjecting and measuring the rate of flow of the decarburizing gas(es)into the reactor vessel are provided. Suitable gas injection devices aretuyeres, surface lances,

submerged lances, etc. Input gas flow rates can be determined by suchmeans as flow-meters, orifice plates, etc. The composition of the inputgases can be generally obtained with gas-analyzing devices such as amass spectrometer. Similarly, techniques are available for thedetermination of olf-gas compositions. One such technique, whichgenerates a continuous analysis, is the subject of several Frenchpatents [Nos 1,309,212 (Oct. 8, 1962); 1,325,024 (Mar. 18, 1963)]. Themethod has been published in the Journal of Metals, June 1964, p. 508,and is generally familiar to steel making artisans. It involvescontinuous determination of the carbon monoxide and carbon dioxidecontents of the effluent gases from the rening vessel. Suchdeterminations then serve as an indicator of the carbon content anddecarburization rate of the melt. The input and off-gas analyses, usedas the input for a properly calibrated computed device, yield aninstantaneous indication of the processes occurring in the melt.Sampling of the Gif-gases is difficult since atmospheric air entrainedat the mouth of the reactor vessel results in immediate combustion ofthe off-gases. However, satisfactory samples can be obtained by takingprecautionary measures, such as t-hat described by M. Allard et al. inthe Journal of Metals, June 1961, p. 421. By controlling the pressurebetween the exhaust hood and the mouth of the reactor vessel, astationary combustion zone is created for the off-gases, and since thepressure condition prevents an air draft into the area of the vesselmouth virtually all the evolved gases can escape in their pure form intothe hood where they can be sampled and analyzed.

In the preferred embodiment, the process of the present inventionemploys continuous inputand off-gas analysis for the purpose ofindicating the eiciency of oxygen consumption by melt carbon. Thecarbon-oxygen reaction occurs in preference to metallic oxidation if thebath carbon is equal to or above the equilibrium level for the system inquestion, and if the carbon available for oxidation is at leaststoichiometrically balanced by the injected oxidizers. With the aid of acontinuous gas analyzer such as described above, the oxygen equivalentof the effluent gases (where oxygen equivalent is given by the sum ofunreacted oxygen and the oxygen contents of evolved CO and CO2) iscompared with the injected and entrained gaseous oxygen. A lower valuein the effluent stream (and therefore an efficiency of less than 100%)implies that a proportion of the supplied oxidizers is consumed formetallic oxidation, with only a fraction reacting with melt carbon toyield the analyzed carbon oxides. At every stage of decarburization anyimbalance is immediately detected and can be corrected by altering oneor more of the factors listed previously in a manner to be elaboratedupon in the following discussion.

(a) Dynamic control using mixed gases Since the carbon content that canbe attained without metallic (c g. chromium) oxidation duringdecarburization at a given temperature is a function of the partialpressure of carbon monoxide in the evolved gas bubbles, it is apparentthat the CO partial pressure can be varied by incorporating in theinjected gas stream an inert component which does not enter into anychemical reaction. The inert gases as well as hydrogen and nitrogen canusefully serve this purpose during decarburization. Active oxidizinggases that are generally employed for carbon removal are oxygen, steamand carbon dioxide. By adequate adjustment of the proportions of thesegases in the input stream, CO at a predetermined partial pressure can begenerated.

The process of dynamically-controlled mixed gas decarburization ofsteels can be suitably performed in a reactor such as a BOF or othercontainer equipped with a means for input-gas and off-gas measurements.The molten steel is tapped into this vessel and held at a knowntemperature. FIG. I is an illustration of a typical process employingmixed gases. Prior to its entry into the vessel, the

gas is measured as to ilow rate and analyzed to give an instantaneousreading of its composition. Simultaneously, a sample of the gaseousreaction product is analyzed as previously described, and its oxygenequivalent determined. Assuming carbon-oxygen reaction eiciency, as wellas a constant melt temperature of 3000 F., the results of FIG. I areobtained for oxygen-argon decarburization of an 18% chromium steel bath.If, for example, at time t, the input gas is analyzed at greater than40% oxygen, the decarburization will continue as shown on the graph, butin addition, the excess oxygen will be applied towards metallicoxidation. This condition will be immediately indicated by a drop incarbon-oxygen reaction eiciency to a value below 100%, and this in turnwould call for an increase in the proportion of inert component of theinput gas stream. When the balance has been restored, the processefliciency will revert to 100%. The illustrated process assumes aconstant total gas flow rate and constant temperature. Deviation fromthese ideal conditions does not alter the basic concepts described. Ineither case, input and oE-gas analysis serves as an indicator of theeiliciency of the reaction.

The example illustrated in FIG. I is a process involving stepwisereductions in the oxygen content of the injected gas stream. By adoptinginnitesimally small steps, a continuous curve is obtained. Such a curveis adaptable to suitable control devices to supply mixed gases accordingto the specified schedule. The process according to this schedule is themost efficient since it involves the use of the minimum quantity ofinert gases for the amount of carbon removed, and also results in theminimum process time.

(b) Dynamic control through ambient pressure variations The carbonmonoxide partial pressure in the evolved gas bubbles, and consequently,the attainable carbon level in equilibrium with a given bath chromiumand temperature, can be varied by means of the ambient pressure aroundthe melt. A pressure-controlled process requires the gas injection,metering and analyzing devices described in (a) above. But in addition,the reactor vessel is equipped with a lid to facilitate its evacuation.The means of evacuation can be pumps, ejectors or any suchvacuum-generating equipments. Unlike mixed-gas decarburization, thedecarburization process can be accomplished using either oxygen alone oroxygen-inert gas mixtures.

A pressure reduction sequence along with the decarburization bath at3000 F. is illustrated in FIG. l1 for a one ton bath of 18% chromiumsteel employing 30 s.c.f.m. of oxygen. The scheme illustrated assumesthat the gas pumping capacity of the evacuation equipment is unlimitedat all pressure ranges. If the pumping capacity of the system involveddecreases as the pressure within the vacuum chamber decreases, acontinuous reduction of the decarburizing gas flow rate in accordancewith the capacity of the pumps is required.

As in example (a), an increase of the melt ambient pressure results inboth chromium oxidation and reduction of reaction efficiency. Such asituation is immediately sensed by the off-gas analysis and compensatedfor by the commensurate amount of pressure reduction until optimumeciency is re-established. Also as in example (a), the pressurereduction when made smoothly continuous, rather than in steps, furtherincreases the efficiency of the process and reduces the process time.

(c) Dynamic control through temperature variation The application ofthis techni-que is predicated on the fact that at a given carbonmonoxide partial pressure and bath chromium content, a lower carboncontent can be attained in a bath at higher temperatures. The dynamicvariation of temperature employs the same gas injection, metering, andanalyzing devices as previously described. Either oxygen oroxygen-diluent gas mixtures can be employed. Temperature variations areachieved by one or more of the following means: induction heating of thebath, gas heating with carbonaceous fuels such as employed in the openhearth, plasma heating, electron beam heating, injection of oxygen intothe bath to react exothermically with an element such as silicon whichis easier to oxidize than carbon, or by any other suitable means. Thebath temperature can be continuously monitored with the aid of suchdevices as thermocouples or optical pyrometers. In those instances wherecarbonaceous fuels are used, the olf-gas analysis includes theadditional carbon oxides due to the heat source. The proportion of thiscomponent in the off-gases is computed from a knowledge of thecombustion rate on the bath.

FIG. III illustrates a decarburization process, for a one ton 18%chromium heat, decarburized with oxygen at 30 s.c.f.m., and whichemploys the concept of dynamic control via temperature variations. As inthe previous illustrations, an imbalance between input and output oxygenis indicated by the gas analyzing devices. Such irnbalance, denoted byan oxygen utilization efliciency of less than 100%, is then off-set byan increase in melt temperature accomplished by one or more of the meanspreviously indicated.

The application of temperature-controlled dynamic balance indecarburization, as in FIG. III, requires a reactor vessel having alining capable of withstanding the relatively high temperaturesindicated. In practice however, the lower the range of temperaturesinvolved, the more economical the process and lower temperatures arepossible by using a combination of temperature, ambient pressure and gasmixture controls.

(d) Dynamic control through gas-metal surface area variations It iscommonly known that the carbon-oxygen reaction occurs at the metal-gasinterface. Furthermore, the rate of decarburization in steels isdirectly proportional to the gasmetal surface area. The efficiency ofoxygen consumption, therefore, can be controlled by varying theavailable gasmetal contact area.

The gas analyzing and efficiency determining devices employed in theapplication of this embodiment are the same as those describedpreviously. In addition, means for varying gas-metal contact areainclude (1) sub-surface gas injection resulting in the generation ofsmall gas bubbles which furnish a large surface area, (2) mechanical or(3) induction stirring to continuously expose fresh metal surface to thedecarburizing gases, etc.

It is apparent from the above that various changes and modifications arepossible in practicing the invention. Thus, for example, it may besometimes desirable, usually in making plain carbon steel, to allow somemetal oxidation. In alloy steel making, it is generally desirable toavoid oxidation of expensive alloying elements. A convenient techniqueto measure carbon removal efliciency is by the following determination:

Percent carbon removal efiieieney Rate of oxygen consumption by carbon100 Rate of total input oxygen To avoid any metallic loss, the carbonremoval efiiciency must be equal to or greater than 100%. If some metalloss is tolerable, then this factor can be some predetermined lowervalue, such as -85%.

I claim:

1. In the method of decarburizing molten metal wherein oxidizingmaterial consisting essentially of oxygen and oxygen-containing gas isintroduced to steel to react with carbon contained therein, theimprovement which comprises controlling decarburization to substantiallypreclude undesirable loss of metal values resulting from fluctuatingthermodynamic conditions by supplying said oxidizing material, measuringthe rate of carbon removal from the molten metal, measuring the rate ofoxidizing input, and continuously maintaining a kinetic balance betweenthe aforementioned rates by at least one of the following: (l) supplyingsaid oxidizing material in admixture with diluent gas and substantiallycontinuously monitoring and adjusting as necessary the proportions ofoxidizing material and diluent gas and (2) substantially continuouslymonitoring and adjusting as necessary the ambient pressure under whichdecarburization is occurring.

2. An improvement according to claim 1 wherein the diluent gas is atleast one from the group consisting of inert gases, hydrogen, carbonmonoxide, carbon dioxide and steam.

3. An improvement according to claim 1 wherein the proportion of diluentgas is increased in response to an indication of a drop in theefficiency of oxygen consumption by carbon to a value below 4. Animprovement according to claim 1 wherein the ambient pressure isdecreased in response to an indication of a drop in the efficiency ofoxygen consumption by carbon to a value below 100% 5. An improvementaccording to claim 1 including: supplying oxidizing gas in admixturewith diluent gas for decarburization, continuously monitoring andadjusting as necessary the proportions thereof and continuouslymonitoring and adjusting as necessary the ambient pressure under whichdecarburization occurs.

6. An improvement according to claim 1 wherein the diluent gas is argon.

References Cited UNITED STATES PATENTS 2,803,535 8/1957 Kosmider et al.75-60 2,855,293 10/ 1958 Savard et al. 75-60 3,003,865 10/1961 Bridges75--60 3,046,107 7/ 1962 Nelson et al 75-59 3,252,790 5/ 1966 Krivsky75--60 3,307,937 3/ 1967 Philblad et al. 75-60X 3,372,023 3/ 1968Krainer et al 75-60 3,377,158 4/1968 Meyer et al. 75-60 3,432,288 3/1969Ardito et al 75-60 L. DEWAYNE RUTLEDGE, Primary Examiner G. K. WHITE,Assistant Examiner U.S. Cl. X.R. 75--49

